Our current understanding of the cognitive processes underlying human learning enables cognitive psychology to offer valuable guides to the design of curricula in school subjects. This article summarizes some principles of curriculum design drawn out of this literature that have been applied very successfully to middle-school instruction in mathematics and science, using the general plan of learning from examples and by doing. The article does not claim unique efficacy for this specific method, but shows how experience gained from employing the theory of adaptive production systems provides concrete practical advice for achieving effective learning with understanding.